Molluscs: Pila | Zoology Optional Notes for UPSC PDF Download

Introduction to Pila

Habit and Habitat of Pila

• Pila globosa thrives in diverse aquatic environments, including ponds, pools, tanks, lakes, and rice fields.
• It can also adapt to freshwater streams, rivers, and even brackish waters with low salinity.
• Being herbivorous, Pila globosa is particularly abundant in areas with succulent aquatic vegetation.
• These snails are amphibious, capable of life in both water and on land.
• They possess a dual respiratory system, utilizing ctenidium for underwater respiration and a pulmonary sac for respiration on land.
• Pila demonstrates an ability to aestivate during prolonged droughts and return to normal activity during rainy periods.
• When disturbed, Pila retracts into its coiled shell and seals the opening using its operculum.

Structure of Pila

• Pila's body is enclosed in a thick, spirally-coiled, univalve shell.
• The shell takes the shape of an elongated cone coiled around a central axis, with each spiral revolution referred to as a whorl.
• The apex of the shell is the oldest part, while the penultimate whorl and body whorl are larger to encompass most of the body.
• The last whorl contains a sizable aperture, which can be sealed by the operculum.
• The operculum, a flat calcareous plate, is secreted by cells in the foot and has an elliptical boss for opercular muscle attachment.
• The type of coiling in Pila globosa is right-handed and is known as dextral, in contrast to the rare left-handed sinistral coiling.

Microscopic Structure of the Shell:

• The shell consists of three distinct layers: the chitinous periostracum (outermost layer), calcareous ostracum, and a thin calcareous hypostracum.
• The innermost layer, the nacreous layer, is smooth and glossy.
• The periostracum may appear as a homogeneous membrane in adults, lacking the characteristic bands seen in young individuals.

Anatomy of the Body:

• Pila's body can be divided into the head, foot, and visceral mass.
• The head extends into a partly contractile snout and features two pairs of tentacles.
• The longer pair is filament-like and hollow, with bead-like eyes at the base of each tentacle.
• The shorter pair of tentacles, known as labial palps or the first tentacle, serves as an anterior extension of the snout.
• Fleshy projections called nuchal lobes or pseudoepipodia are found on the sides of the head, with the left nuchal lobe forming a respiratory siphon.
• The foot, which is large and muscular, has a broad, flat, and smooth sole adapted for crawling. It contains pedal glands.
• The visceral mass, coiled similarly to the shell, houses the main organs of the body and is covered by the mantle or pallium.

Mantle and Pallial Cavity

• The mantle serves three key functions: protection of the visceral mass and head, additional respiration, and shell secretion.
• The mantle is free anteriorly and encloses a spacious cavity called the pallial or mantle cavity.
• This cavity houses the visceral organs of the snail and is imperfectly divided into left and right chambers by a longitudinal ridge known as the epitaenium.

Position of Apertures:

• The mouth and anus are located on the same side of the body.
• Both the anal and genital apertures are situated on the right mantle opening.

Integumentary System of Pila:

• Pila's skin consists of an outer epidermis and an inner dermis.
• The epidermis, covering the entire body, is a single layer of epithelium, with some cells modified into unicellular glands responsible for secreting mucus, pigment, and lime.
• The dermis or corium includes muscle fibers and connective tissue, along with a notable presence of pigment cells.

Body Cavity of Pila:

• In adults, the primary body cavity is the haemocoel, while the true coelom is greatly reduced.
• The coelom is represented by the pericardial cavity and the cavities surrounding the kidney.

Locomotion in Pila

• Pila's foot is crucial for locomotion.
• The flat sole of the foot enables slow movement by creeping on surfaces.
• The extension of the foot is facilitated by an influx of blood into it, which causes it to protrude through the shell opening.
• The foot contains vertical, longitudinal, and transverse muscles, resulting in wave-like contractions on its surface during movement.
• Vertical muscles drive surface contractions, transverse muscles propel the foot forward, and longitudinal muscles pull the posterior end of the foot to facilitate movement.

Digestive System of Pila

• Pila is herbivorous, primarily feeding on aquatic vegetation.
• The digestive system comprises a tubular digestive canal and digestive glands.
• The digestive canal consists of three distinct regions: the fore gut, mid gut, and hind gut.

(i) Fore Gut:

• Includes the buccal mass and the esophagus.
• The mouth leads to the buccal cavity, enclosed by the muscular buccal mass.
• A pair of chitinous jaws guard the entrance of the mouth, while a ribbon-like structure called radula lies at the floor of the buccal cavity.
• The radula contains transverse rows of teeth used for rasping food.
• The anterior part of the buccal mass has jaws with serrated edges to cut aquatic vegetation.
• Salivary glands, situated on the posterior side of the buccal cavity, secrete mucus and a starch-digesting enzyme.

(ii) Mid Gut:

• Consists of the stomach and the intestine.
• The stomach is red in color and bent in a 'U'-tube shape, with a cardiac chamber at one end and a pyloric chamber at the other.
• The cardiac chamber constitutes the main part of the stomach, with a corrugated wall, while the pyloric part exhibits transverse folds.
• The long, narrow intestine forms 2.5-3 coils and connects the stomach to the rectum.

(iii) Hind Gut:

• Includes the rectum, a thick-walled tube situated in the right side of the mantle cavity.
• The anus is located near the mouth within the right mantle opening.
• Digestive glands comprise the salivary gland and hepatic gland/digestive gland liver.
• Salivary glands secrete mucus and starch-digesting enzymes, and the hepatic gland constitutes the main bulk of the visceral hump.

Respiratory System of Pila

• Pila is amphibious, utilizing both aquatic and atmospheric oxygen.
• It has a double mode of respiration, absorbing oxygen from the water via ctenidia (gills) and from the air through the pulmonary sac.

Circulatory System of Pila

• Pila's circulatory system is well developed and complex due to its dual mode of respiration involving both gills and a lung.
• The system comprises the heart, pericardium, arteries, veins, and sinuses.
• The heart is situated in the visceral whorl, and it consists of an auricle and a ventricle.
• Blood is distributed to different parts of the body by the cephalic and visceral aortae, which branch into various arteries.
• Various sinuses collect and transport blood to the auricle for distribution through the ventricle.
• The blood of Pila contains amoeboid cells and a blue, copper-containing respiratory pigment called haemocyanin.
• Circulation includes both aerial and aquatic phases, with blood directed to the pulmonary sac during aerial respiration and the ctenidium during aquatic respiration.

Excretory System of Pila

• The excretory organ in Pila is the kidney, consisting of anterior and posterior renal chambers.
• The anterior renal chamber is oval and lies in front of the pericardium in the mantle cavity.
• The posterior renal chamber is situated behind the anterior chamber, between the rectum and the pericardium.

• The two renal chambers are interconnected and filter nitrogenous waste from the blood.
• The excretory fluid is eventually eliminated from the body through the right siphon, along with outflowing water.
• Pila adapts to water conservation during terrestrial phases by converting ammonia into insoluble uric acid.

Nervous System of Pila

• Pila's nervous system consists of ganglia, commissures, connectives, and nerves.
• Sense organs include the osphradium, tentacles, statocyst, and eyes.
• Pila exhibits sexual dimorphism, with males having a more developed copulatory organ compared to females.

Development of Pila

• Development in Pila is direct, with no parental care.
• The embryo floats in liquid albumen surrounded by solid albumen.
• During development, torsion occurs, leading to the asymmetry of the body.
• The hatched young snails resemble the adult form.